The present invention relates to double side grinding process and apparatus for thin disklike work, and more particularly to a process and an apparatus for simultaneously grinding opposite surfaces of thin disklike work such as semiconductor wafers.
Apparatus for grinding opposite surfaces of work at the same time are already known wherein the work as placed in a pocket (hole) of a rotating disklike carrier is passed between a pair of grinding wheels having grinding faces provided by their end faces and opposed to each other. In this case, the grinding faces of the wheels must be greater than the work in outside diameter. The carrier is usually provided with a plurality of pockets formed on a circumference close to its outer periphery and equidistantly spaced apart. A portion of the carrier is also positioned between the pair of grinding wheels along with the wafer. The thickness of this portion of the carrier of course needs to be smaller than the clearance between the pair of grinding wheels during grinding, namely, the thickness of the work as finished.
The semiconductor wafers presently available include those having an outside diameter of about 200 mm (8 inches) and those with an outside diameter of about 300 mm (12 inches), and are all about 0.8 mm in thickness (as finished by grinding). Thus, the thickness is very small as compared with the outside diameter. In the case where such wafers are to be ground by the apparatus described above, the grinding wheels have a large outside diameter, and the carrier which rotates with the wafer held thereon also has a large size since the wafer is relatively large in outside diameter, consequently making the apparatus large-sized. Further because the wafer has a small thickness, the portion of the carrier to be positioned between the grinding wheels along with the wafer needs to have a greatly reduced thickness. Although a grinding force acts on the carrier positioned between the grinding wheels, especially on the pocket portion thereof, through the work accommodated in the pocket, this portion will have a lower strength if reduced in thickness, presenting difficulty in moving the work smoothly. For this reason, it has heretofore been difficult to grind opposite surfaces of wafers.
The same problem is encountered also with thin disklike work other than wafers.
To overcome the above problem, the present applicant has proposed a double side grinding apparatus for thin disklike work which apparatus comprises a pair of rotatable annular grinding wheels having opposed annular grinding faces provided by respective end faces thereof and so arranged as to be movable relative to each other axially thereof, and work rotating means for rotating the thin disklike work about its own axis while supporting the work in a grinding position between the grinding faces so that opposite surfaces of the work to be worked on face the respective grinding faces of the pair of wheels, with the outer periphery of the work intersecting the outer periphery of each grinding face and with the center of the work positioned inwardly of the grinding faces [see JP-A No. 10-128646 (1998)].
With this apparatus, the pair of grinding wheels are usually so arranged that the opposed grinding faces are positioned in parallel to each other. Thin disklike work is ground over opposite surfaces thereof in the following manner. With the work rotated about its own axis in the grinding position, the pair of grinding wheels are rotated and moved toward each other, whereby the grinding faces are brought into contact with the respective corresponding work surfaces to advance each grinding face to the position of a predetermined depth of cut. The grinding wheels are stopped from advancing in the directions of depth of cut for spark-out grinding and thereafter moved away from each other to separate the grinding faces from the work surfaces.
With this apparatus, the entire surfaces of the work pass between the grinding faces in contact therewith while the work makes one turn of rotation about its center, with the outer periphery of the work intersecting the outer peripheries of the grinding faces and with the center of the work positioned inwardly of the grinding faces, whereby both work surfaces can be entirely ground at the same time.
However, the portion of the work other than the portion thereof in the vicinity of its center comes into contact with the grinding faces only during a portion of the time taken for each turn of rotation of the work, whereas the central portion in the vicinity of the center is in contact with the grinding faces at all times. Accordingly, the central portion is greater than the other portion in the amount of grinding. This results in the problem that the work as ground has an increased thickness toward its outer periphery and a reduced thickness in the vicinity of its center, hence great variations in the thickness of the work.
An object of the present invention is to overcome the foregoing problems and to provide double side grinding process and apparatus for thin disklike work which ensure diminished variations in the thickness of work as ground.
The present invention provides a process for grinding opposite surfaces of thin disklike work simultaneously with annular grinding faces of ends of a pair of grinding wheels arranged as opposed to each other, the process being characterized by bringing the grinding faces into contact with the respective work surfaces to advance each grinding face to the position of a predetermined depth of cut by moving at least one of the grinding wheels while rotating the grinding wheels and rotating the work about an axis thereof as supported in a predetermined grinding position between the grinding wheels so that an outer periphery of the work intersects outer peripheries of the grinding wheels with a center of the work positioned inwardly of the grinding faces, stopping each of the grinding wheels from advancing in the direction of depth of cut, moving each of the grinding wheels and the work relative to each other in a direction parallel to the work surface until the center of the work is positioned externally of the grinding faces and separating the grinding faces from the work surfaces.
The grinding wheels are rotated at a higher speed than the work. Preferably, each of the grinding wheels is stopped from advancing in the direction of depth of cut after advancing to the position of a predetermined depth of cut to start spark-out grinding, and each grinding wheel and the work are moved relative to each other in a direction parallel to the work surface before the spark-out grinding operation is completed. However, simultaneously when the grinding wheels are stopped from advancing in the directions of depth of cut after advancing into the work depthwise at a very low speed, each grinding wheel and the work can be moved relative to each other in a direction parallel to the work surface to be worked on. Further alternatively, the spark-out grinding operation is continued after each grinding wheel and the work are stopped from moving relative to each other, and each grinding face is moved away from the work surface after the spark-out grinding operation is completed, or simultaneously when each grinding wheel and the work are stopped from moving relative to each other, the spark-out grinding operation is terminated, and each grinding face is moved away from the work surface. Each grinding face may be moved away from the corresponding work surface by moving each grinding wheel and the work relative to each other until the work is brought out from between the pair of grinding wheels.
The work surfaces to be worked on are ground by advancing the grinding wheels in rotation toward the directions of depth of cut, with the grinding faces thereof in contact with the respective work surfaces. The entire surfaces of the work pass between the grinding faces in contact therewith while the work makes one turn of rotation about its center, with the outer periphery of the work intersecting the outer peripheries of the grinding faces and with the center of the work positioned inwardly of the grinding faces. Accordingly, both work surfaces can be entirely ground at the same time by merely rotating the work about its center in this arrangement when the grinding faces of the wheels have an outside diameter which is slightly greater than the radius of the work. The work needs only to be rotated about its center in this arrangement although it is conventionally necessary to move the work with use of a carrier or the like. Even when in the form of a thin disk, the work can therefore be ground with ease reliably using a compacted apparatus. The work surfaces can be entirely ground by using grinding wheels having grinding faces with an outside diameter slightly greater than the radius of the work, and there is no need to use great grinding wheels whose grinding faces are greater than the work in outside diameter. This also serves to make the apparatus compact.
If the center of the work is positioned externally of the grinding faces, the portion of the work in the vicinity of its center is held totally out of contact with the grinding faces. Accordingly when each grinding wheel is advanced to the position of a predetermined depth of cut and thereafter stopped from advancing in the direction of depth of cut, and if each grinding wheel and the work are then moved relative to each other in a direction parallel to the work surface to be worked on until the center of the work is positioned externally of each grinding face, only the portion of the work other than the vicinity of the center of the work is ground, with the work portion in the vicinity of the center held out of contact with the grinding faces. The work ground is therefore diminished in the difference between the thickness of the portion of the work in the vicinity of its center and the thickness of the other portion of the work, hence diminished variations in the overall thickness of the work.
Thus, the process of the present invention makes it possible to simultaneously grind both surfaces of thin disklike work easily by a compact apparatus, with diminished variations in the thickness of the work ground.
Each of the grinding wheels and the work are moved relative to each other in a direction parallel to the work surface, preferably with the work rotated at a lower speed than in the preceding grinding operation.
Each of the grinding wheels and the work are moved relative to each other in a direction parallel to the work surface preferably by moving the work in a direction parallel to the work surface.
When to be moved, the pair of grinding wheels need to be moved while being held positioned relative to each other in a definite relationship with high accuracy, so that it is difficult to move each grinding wheel and the work relative to each other. However, if the work is made movable as described above, there is no need to move the grinding wheel, with the result that the wheel and work are easily movable relative to each other.
The present invention provides an apparatus comprising a pair of rotatable grinding wheels having opposed annular grinding faces at respective ends thereof and so arranged as to be movable relative to each other axially thereof, work rotating means for rotating thin disklike work about an axis thereof while supporting the work in a grinding position between the grinding faces so that opposite surfaces of the work to be worked on face the respective grinding faces of the wheels, and moving means for moving each of the grinding wheels and the work rotating means relative to each other in a direction parallel to the surfaces of the work supported by the rotating means, the apparatus being characterized by bringing the grinding faces into contact with the respective work surfaces to advance each grinding face to the position of a predetermined depth of cut by moving at least one of the grinding wheels while rotating the grinding wheels and rotating the work about an axis thereof as supported in a predetermined grinding position between the grinding wheels so that an outer periphery of the work intersects outer peripheries of the grinding wheels with a center of the work positioned inwardly of the grinding faces, stopping each of the grinding wheels from advancing in the direction of depth of cut, moving each of the grinding wheels and the work relative to each other in a direction parallel to the work surface until the center of the work is positioned externally of the grinding faces and separating the grinding faces from the work surfaces.
The work is rotated about its own axis by the work rotating means, as thereby supported in the grinding position, and the pair of grinding wheels are rotated at a higher speed than the work. At least one of the grinding wheels is moved in this state, whereby the grinding faces are brought into contact with the respective work surfaces and advanced each to the position of a predetermined depth of cut, with the outer periphery of the work intersecting the outer peripheries of the grinding faces and with the center of the work positioned inwardly of the grinding faces. With each of the grinding wheels stopped from advancing in the direction of depth of cut, each grinding wheel and the work are thereafter moved by the moving means in a direction parallel to the work surface until the center of the work is positioned externally of the grinding faces, and the grinding faces are separated from the work surfaces.
In this way, the foregoing process of the invention can be practiced by the apparatus of the invention, with the result that both surfaces of the thin disklike work can be ground easily at the same time as previously described, while it is possible to compact the apparatus and to diminish variations in the thickness of the work ground
Preferably, the moving means moves the work in a direction parallel to the work surface to thereby move each of the grinding wheels and the work relative to each other in a direction parallel to the work surface.
The grinding wheel and the work can then be moved easily relative to each other as previously described.